The invention relates to a method of making highly adsorptive polymers, specifically by polymerization of monomers by the precipitation polymerization technique.
In the art of making polymeric particles capable of entrapping fluids and/or solids (actives), there exists a few different techniques that are usually used. One such technique entails polymerization in the presence of actives giving monolithic articles where the actives stay in the monolithic article (in situ polymerization) or are extracted, leaving macroporous cross-linked polymeric structures. In the latter case, the actives, since they are forming pores, are sometimes called porogens. After extraction, the rigid porous polymer can be refilled by other actives. The porogen is soluble in the monomers, but does not swell the resulting polymer. Dependent on the physical properties of the porogen, the formation of the macropores can be achieved by proper (increased) cross-linking.
A very convenient method of preparation of both of these systems is suspension polymerization in a medium (solvent) which does not dissolve the monomers and actives. Usually for hydrophobic systems, the solvent is water, and for hydrophilic systems, the solvent is cyclohexane, benzene, toluene, xylene, chlorobenzenes, chloroethanes, etc. Protective colloids such as polyvinylpyrolidone, polyvinylalcohol, magnesium hydroxide, calcium phosphate, etc. are used to prevent the coalescence of the beads during polymerization.
Both of the above mentioned systems are "in situ polymerization" techniques, using the same principle in that the polymerization is carried out in the presence of the active or porogen. The only difference between these systems is that, in the case of the actives, the product is used as such, while in the case of the porogen, the porogen is usually extracted and replaced by another material. The procedure of extracting and replacing is very often tedious, adding cost to the manufacturing.
In situ polymerization is carried out by dissolving the monomers in a functional material to form a uniform solution, and, thereafter, inducing polymerization. The functional material must be a solvent for the monomers, but not a swelling agent for the polymer. The resulting product of in situ polymerization is a solid material formed of a polymeric lattice entrapping the functional material. The amount of functional material used is such that when polymerization is complete, the functional material is substantially wholly contained within the polymeric lattice and does not exist freely in the product. If the functional material is extracted from the polymeric lattice, the resulting polymer is in the form of hard, porous beads.
The third method of making polymers capable of entrapping other materials is by precipitation polymerization. The resulting product from precipitation polymerization is generally a polymer in the form of a powder and the powder is a combined system of particles. The system of powder particles includes unit particles, agglomerates of fused unit particles, and aggregates of clusters of fused agglomerates. The particles themselves are not generally highly porous. This invention describes an improved method for making such powders using precipitation polymerization.
As described, the type of polymerization technique used is an important factor in the determination of the resulting product. In addition, within each type of polymerization, there are procedural alternatives which, too, can have a significant impact on the resulting product. The differences in the polymerization techniques are enough that a procedure used in one type of polymerization technique will not necessarily have the same effect if used in another polymerization technique. Therefore, techniques and procedures are carefully selected based on the product properties desired.
With respect to the prior art, however, there remains a need for an easy, consistent method of making submicron size, highly adsorptive, powder-type polymers which are of a soft nature suitable for cleaning scratchable surfaces, such as contact lenses.